Not too hard to do - but extrememly expensive and you won't be happy with the results. Running a constant 250 watts off of conventional automotive style batteries is going to mean switching them and charging them every morning, and you must not have 120v power near the batteries, or you would just run the lamp off them, so I guess you would have to disconnect the batteries and lug them back to the house or shop or somewhere to charge them up

I am not recommending you go this route, - but this is what you would need to power a 250 watt heat lamp for 24 hours.

1) 12volt to 120volt inverter (500 watts `100$+)
You will need a 12v to 120v inverter. The price on these is getting cheaper all the time, and a lot of automotive type stores are carrying them now, quality is sometimes a problem, but a heat lamp doesn't need particulary clean power. If you are using a 250 watt heat lamp, you would want to go with a 500 watt inverter at least, most often now the rating on the inverter doesn't reflect the true constant duty cycle.

2) Batteries (22 car batteries 1000$ +)
250 watts at 12 volts is 21 amps, but there is some loss in the inverter, so I would figure for at least 23 amps constant draw on the battery bank. for 24 hours, you would draw 552 amp hours out of your battery bank. You never want to draw your battery bank down more than 50%, and if you want it to last - you will only want to draw it down about 25%, but lets use the more aggressive 50%, you would need a battery bank of at least 1100 amp hours to run your lamp for 24 hours, but then they will need to be charged, so you will need a matching set to swap out while you charge the first set. Figure on at least 2200 amp hours worth of batteries. Your basic 12 volt car battery has about 100 amp hours of capacity, so figure at least 22 of those, plus all the cables to hook them together and to the inverter. If you went with a larger inverter specific battery, you might get up to 250 amp hours out of a large 8D (200+ pounds each!) battery, but very expensive as well.

3) Battery Charger (at least 60 amps 100% duty - at least 100$)
Then you will need to charge up your depleted bank. It was drawn down 50% last night, so you haul the 11 batteries into the back of your truck and take them to your shop and hook the up to the charger, you want them charged by the next morning, so you need a charger that can put the 550 amps back into the batteries in about 20 hours, thats only a 25 amp charging rate, but charging isn't 100% efficient either, so you will need at least 30 amps of constant charging current average to charge those batteries up. Lead-Acid batteries tend to charge hard at first then take a while to trickle up to capacity, so I would figure get at least a 60 amp constasnt duty 12 volt charger. (expensive)

4) Power cables and terminals (100$ +)
Daisy-chaining batteries together to make a big bank gets expensive fast too. You need large enough gauge cable to handle the fast charge rate, which should be higher than the rate of draw off from the inverter. Figure at least 6 guage copper wire to be safe, plus all the terminals to connect to the batteries and the cost of making the cables up.

Look for a better solution, how about a generator with a long extension cord to get the exhaust away from the brooder?

And your chickens will probably knock it over and burn down the neighborhood.

Run an orange cord and use a single 250w red heat lamp plugged into a thermocube or a mechanical lamp timer, securely attached at least 2' from anything in every direction.

that's why it has an O2 sensor, i would have suggested an LP but seeing that there is a distance issue propane is alot more portable. Each to his own though everyone has differences in opinion or preferences

Where I want to house the chicks I do not have a power source (we're working on that but its too far for an extension cord). It stays pretty warm where I am and the barn they are in holds heat extremely well so the daytime is not a problem. Night time is not bad, but I worry about a temperature drop which we have had sometimes in the last few weeks. It sounds like I'm going to have to watch the weather and just bring them to the porch in case it gets too cool one night.

Perhaps what you need is an insulated hover, lined with reflective foil perhaps, that they can shuffle under all together and it will hold their heat around them. This is an old-timey way of minimizing the amount of heat needed for raising chicks. You can find designs in old poultry books; or just make something like Robert Plamondon's insulated hover brooder (go to www.plamondon.com, go to poultry articles section, and hunt around til you find it) and simply omit the actual lightbulbs.

If the barn is basically warm enough most of the time, I think you have a pretty reasonable chance of being ok with a well-designed hover (and draft protectors) for occasxional cool nights.

Another option to consider is bringing out a couple of (tightly lidded) large buckets of hot water, just before you go to bed and again at daybreak, to put inside the hover to add warmth and for chicks to huddle against if necessary.

And your chickens will probably knock it over and burn down the neighborhood.

Run an orange cord and use a single 250w red heat lamp plugged into a thermocube or a mechanical lamp timer, securely attached at least 2' from anything in every direction.

that's why it has an O2 sensor, i would have suggested an LP but seeing that there is a distance issue propane is alot more portable. Each to his own though everyone has differences in opinion or preferences

Thermodynamics are not subject to preference or opinion. The heater you link to is fine for warming the work area in a garage, but is utterly inappropriate to heat a chicken coop. Even worse for brooding.

Also, the exhaust contains water. Water vapor in a coop must be vented aggressively. You can cycle enough air to completely negate the use of a "vent free" propane heater just controlling moisture. I know, I've tried to to woodworking in an unheated garage. I nearly froze to death and almost caught my clothes on fire using an "eyeball" type tank-top heater, working with the door open to try to dump moisture to keep my wood from getting too moist.

A 10,000 btu heater is putting out heat roughly equal to 2(two) 1500W heaters, but from a single point-source the size of a paperback novel. That's as much heat as 12 red-lamp 250W brooder heaters. The surface temperature of the heater's radiant surface is around 3000 degrees Fahrenheit. The "comfort distance" will vary depending on air flow and ambient temperature, but in my experience it's about 4' away, with the side of the body away from the heater getting very cold. Chickens are stupid. They will burn themselves with this kind of heater. It's like replacing all the light bulbs in your house with one street light. You'll be blinded in one area, but everything else is dark.

Fires are started in coops every winter just with 250W heat lamps. Search the forum.

Radiant heat is very efficient in creating comfort without needing to raise the air temperature. The problem is the inverse-square law, the size of the coop, and the type and placement of the heater. Ceiling-mounted radiant heat tubes work great - but if you go to Home Depot you'll notice that they're located about 25' above the customer service desk, and are still quite toasty.

It seems the OP needs the heat for chicks, so the heater needs to be of a lower intensity. Depending on the age of the chicks even 250W might be more than needed.